Field of the Invention
The present invention relates to a moulding device, and to a method for manufacturing thermoplastic containers or objects of an elongate shape, having a major axis and a minor axis. The invention is particularly suitable for manufacturing bottles with a cross-section that is substantially elliptical, rectangular or oval, of the type having a body of which one end has a transverse bottom, the other end ending in a neck joined to the body of the bottle by a shoulder, and on which neck a stopper is mounted to close the bottle in a detachable manner. The method can also be used for the manufacture of tube-shaped containers.
Such containers are used in particular for the packaging of products of a liquid-to-pasty consistency, such as used in the field of cosmetics, pharmacy, the parapharmaceutical field, or in the field of the agricultural/food industry.
During the last few years, the bottle-manufacturing sector has witnessed a massive development of containers whose cross-section is of an elongate shape, this mainly for reasons of marketing, an aesthetic appearance, size, or for being held in the hand.
Typically, such containers are generally obtained by blow moulding a tubular blank obtained by extrusion or injection moulding, or in the case of a two-material container, by co-extrusion or duplex injection moulding.
According to the extrusion technique for example, the plastic materials are heated and pressurized by means of a screw-sleeve assembly so as to provide a tubular blank, in the form of a hot parison, by means of an extrusion head. This tubular blank is subsequently taken between two half-moulds which have the inverse shape of the desired container, and the blow moulding allows the shaping of a parison which stretches under the pressure and assumes the shape of the mould. Cooling allows the material to become fixed in the shape thus obtained.
In the case of injection moulding, a preform is made in an intermediate mould whose shape is adapted to the shape of the container to be made. The preform is cooled without blow moulding the material. Subsequently, with a view to obtaining the container in its final shape, it is introduced at the appropriate temperature (typically around 80.degree. C.) into a mould, wherein it is blown so as to give it the final shape of the container.
Conventionally, the mould is arranged in such a way that the joint plane (the plane along which the two half-shells of the mould are joined) is in the median plane passing through the major axis of the bottle. This solution makes it possible to facilitate the removal from the mould. A major drawback of this solution lies in the fact that for a mould of given dimensions, it is only possible to have a relatively limited number of cavities available, so that relatively few bottles can be made simultaneously.
GB-A-2 292 905 describes a method for blow moulding bottles made of a plastic material in a mould whose cross section is of an elongate shape. The mould has two half-shells forming a cavity of a shape complementary to the shape of the bottle. The half-shells are joined along a joint plane situated in the median plane passing through the minor axis of the bottle. On the bottle thus obtained, the joint lines are situated on the large sides of the bottle. As mentioned in this document, these joint lines can be masked in the final product by placing a label on one or the other, or even on both, of the large sides.
This document gives prominence to the fact that more bottles can be moulded at the same time. Thus the productivity of the equipment is substantially increased and, although the mould is more expensive, the difference is quickly compensated by the substantial gain in productivity.
One of the critical parameters of the blow moulding is linked to the problem of the vents in the mould which must makes it possible to remove out of the mould the air trapped between the material and the mould sides. GB-A-2 292 905 is completely silent on this subject. These vents are, however, indispensable for permitting the blow moulding of the container in good conditions. In the Precis de matieres plastiques, Manual of Plastic Materials! 5th Edition, published by NATHAN, page 135, it is mentioned that the mould must be provided with a very efficient system of vents, to ensure that the blow moulding of the objects is easy. If the holes are too small (smaller than 0.1 mm) the evacuation of the air is unsatisfactory. If the holes are too large (a diameter of 0.3 mm), they mark the surface of the object and affect the aesthetic appearance of the final product. Slots arranged in the joint plane, with a thickness of 0.1 mm and a width of 25 mm, or 0.03 mm over a width of 5 mm are mentioned as giving good results. In the other parts of the mould, it is recommended to cut a cylindrical hole and to stopper it with a hexagon. The air can thus pass between the faces of the hexagon and the internal sides of the hole.
Such a configuration is completely satisfactory for moulds whose distance between the longitudinal axis of the mould and the sides of the mould is substantially the same at all points of the mould. This is in particular the case of an object whose cross-section is that of a cylinder of revolution, i.e. a body of revolution of a rectilinear generator line about a closed path (not necessarily a circle). Indeed in this configuration, the material comes into contact substantially simultaneously at all points of the mould.
For an object of an elongate shape, and in particular for a container of an elliptical shape for example, the material comes into contact at first on the sides of the mould situated on the minor axis, and then secondly on the sides of the mould situated on the major axis. Thus by making a hole centered on the major axis, and then by plugging it with a hexagonal element, the air escape will be distinctly inadequate in the region of the major axis of the mould, because of the presence of the hexagon which is also centered on the axis. Offsetting the hole in relation to the axis is not satisfactory from the point of view of the resulting aesthetic aspect. Moreover, such vents are subject to rapid clogging. The operation of cleaning these vents is lengthy, and expensive in terms of machine time. Similarly, the making of slot lines on the major axis of the mould by cutting the wall of the mould is an unduly complex and unduly expensive operation, because of the dimensions which such slots must have. Their maintenance would also be very tricky.
Another solution of these problems of the air escape lies in using a mould made of a porous material, whose structure permits an air escape at every point of the mould. This solution, although attractive, has the drawback of being expensive. Moreover there occurs fast clogging of the pores of the mould, because of the material particles or dust carried by the air during the blow moulding.
Another less expensive solution would lie in making small holes, of a circular cross-section for example, in the side of the mould and filling them with a piece of porous material with a cross-section identical to that of the holes, so as to obturate the hole almost completely. The air could thus escape through the pores of the material of the filler piece. Such a solution, although it would contribute to substantially improving the air escape opposite the major axis of the mould, would however, have the same problems of clogging as those set out above with reference to the porous mould. Indeed, the air escape would very quickly become insufficient, and even completely ineffective.
GB-A-989 179 describes, with reference to its FIG. 9, a method for moulding a bottle with an elongate cross-section wherein the joint plane is orientated perpendicularly to the major axis of the bottle, that is to say, in the median plane of the bottle passing through the minor axis of the bottle. Such a configuration of the moulding is also suggested in GB-A-935 183. These documents are also completely silent regarding the problem of the vents necessary for the blow moulding.
Thus this problem does not, to date, have a really satisfactory solution.
Thus it is one of the objects of the present invention to provide a method for blow moulding a container with an elongate cross-section in a mould that does not have the above mentioned drawbacks with reference to the known devices of the prior art.
It is, in particular, an object of the present invention to provide a solution which is simple, economical, efficient, and durable in the course of time, to solve the problem of the air escape during the blow moulding of a container of an elongate shape in a mould.
Yet another object of the invention is to enable the operation of a mould allowing the blow moulding in accordance with the invention.
Other objects of the invention will become apparent in detail in the description that follows.
The Applicant has surprisingly discovered that the problem could be solved advantageously by making each half-shell in the form of at least two separate parts kept contiguous along at least one joint surface, the surface serving as a support for a system of vents. Thus in each half-shell there is obtained at least one joint surface which intersects the internal surface of the mould substantially at the point (or points) most remote from the longitudinal axis of the mould.
According to one embodiment a "pseudo" joint plane is obtained perpendicularly to the opening plane of the mould, and its sole function is to serve as a support for a system of vents in the form of at least one narrow slot. The terms "pseudo" joint plane reflects the fact that it does not define an opening plane in the sense that is usually understood. Thus according to the invention the mould is made in at least four parts and during its use opens only in two parts, the other two parts being kept together and held tightly against one another by appropriate means.
In accordance with a first aspect of the invention these objects are attained by making a blow moulding mould with a longitudinal axis X, comprising a first half-shell and a second half-shell, the two half-shells being capable of being joined with a view to the blow moulding of an object along a first joint plane, so as to form a cavity with an elongate cross-section having a major axis and a minor axis, the first joint plane being substantially perpendicular to the major axis, the half-shells moving away from one another along a direction perpendicular to the joint plane during the removal of the object from the mould, characterized in that each half-shell comprises at least two separate longitudinal parts, joined along at least one joint surface, each joint surface being delimited by two opposed sides of two adjacent parts, and at least one of the sides having at least one recess so as to define with the other side at least one narrow slot to allow air to escape during the blow moulding of the object along the surface.
Advantageously, each half-shell comprises two separate parts whose two respective sides are kept joined along a second joint plane situated on the major axis and perpendicular to the first, at least one of the sides having at least one recess so as to define in each one of the ends of the mould situated on the major axis, at least one narrow slot orientated along the axis X, so as to allow air to escape along the major axis.
A plurality of recesses may be regularly distributed over the height of the mould, the recesses being obtained alternately on one and the other of the sides delimiting the joint surface (or surfaces).
Alternatively, the plurality of recesses is distributed regularly over the height of the mould, the recesses being disposed on only one of the sides delimiting the joint surface (or surfaces).
Advantageously, the two half-shells are joined during the blow moulding on either side of the major axis along two respective sides, at least one of the respective sides having at least one recess so as to define, in each one of the mould ends situated on the minor axis, at least one narrow slot orientated along the axis X so as to permit air to escape along the minor axis.
The slots may have a height along axis X of from 3 mm to 25 mm, and preferably from 5 mm to 15 mm, and a width of from 0.01 mm to 0.1 mm, and preferably from 0.02 mm to 0.06 mm.
The mould may comprise a plurality of cavities, so as to mould a plurality of objects simultaneously, the cavities being disposed so that their major axes are disposed side by side, the minor axes being substantially aligned. A first clamping element may be provided to keep the first half-shells of the plurality of cavities joined together, and a second clamping element is provided to keep the second half-shells of the plurality of cavities joined together, so as to permit a simultaneous removal of the said plurality of objects from the mould.
In accordance with a second aspect of the invention, a method is also obtained for blow moulding in the mould with axis X, a thermoplastic object or a container with an elongate cross-section having a major axis and a minor axis, wherein the container is blow moulded in a mould in accordance with the first aspect of the invention.
The thermoplastic container may be formed of a material such as high density or low density polyethylene, polypropylene, polyethylene terephthalate, polyvinyl chloride, or a combination of such materials.
The container may be of the type comprising a body, one end of which comprises a bottom, and the other end ends in a neck joined to the body by a shoulder. Advantageously, the body of the bottle comprises two convex sides along the major axis, so as to facilitate the removal from the mould.
A plurality of containers can be moulded simultaneously during the same moulding operation, the mould defining a plurality of cavities disposed so that the containers have their major axes disposed side by side, the minor axes being substantially aligned.
A third aspect of the invention also concerns a method for manufacturing a thermoplastic container with an elongate cross-section having a major axis and a minor axis, comprising the following steps:
a) obtaining a tubular blank made of a thermoplastic material; PA0 b) introducing the tubular blank into a blow moulding mould; PA0 c) blow moulding the tubular blank so as to apply the thermoplastic material against the internal sides of the mould; and PA0 d) after cooling, removing the container from the mould;
characterized in that the blow moulding mould is according to the first aspect of the invention.
The tubular blank is preferably obtained by extrusion.
The invention also aims to obtain a container by the method of the third aspect.
Thus with the mould used in accordance with the present invention, one obtains the blow moulding of containers, which is improved by a proper disposition of the vents. These vents are easy to make because of their position on a "pseudo" joint plane and can be made in the desired dimensions, however small. The service and maintenance of such a mould are also facilitated by the fact that the cleaning of the vents can be effected quite simply by dismantling the corresponding parts of the mould.